ExtendScript as it's implemented in Photoshop Scripting engine is compliant to:
- ECMA-262 (version 3) - EX4 (EcmaScript for XML)
We don't have native support (among the rest) of JSON, Array.indexOf, Promises, Object.create, etc...
It's becoming increasingly difficult to interface with JS libraries, since shims are not always available/functional.
Also, strategically, we're in a cul-de-sac (aka dead end): there hasn't ever been a major language update (besides ScriptUI changes to support different rendering engines over the years), the PS Scripting language cannot not-evolve now that JS seems to have gained a great momentum and it's as lively as it's never been in the past.
My feature request is not to substitute ExtendScript with JS tout-court (since it would mean give up to EX4 and proprietary stuff), but to incorporate the new ES6 syntax and features - give ExtendScript a place in the future.